PROJECT SUMMARY/ABSTRACT Tooth root cementum plays an essential role in tooth attachment as part of the periodontal complex. Cementum and bone feature comparable extracellular matrix composition and mineral content, however cementum is non- innervated, avascular, and exhibits little or no physiological remodeling, growing by apposition throughout life. Cementum can be compromised by genetic conditions, periodontal diseases, or root resorption, however therapies for periodontal diseases are currently unpredictable, while those for root resorption are non-existent. Critical gaps in knowledge about cementum biology present obstacles to more effectively preventing, abrogating, and reversing periodontal diseases such as apical root resorption. Cementocytes residing in the mineralized extracellular matrix of the cellular cementum exhibit many similarities to osteocytes, mechanoresponsive cells found in bone that direct local bone remodeling based on mechanical loading. It has been speculated that cementocytes may play a regulatory role in cellular cementum formation and/or resorption as osteocytes do in bone, however, at present this question remains uninvestigated and these hypotheses have not been directly tested. Evidence for cementocyte functions in cementum formation or resorption include: 1) Cementocytes present an in vivo and in vitro expression pro?le parallel to osteocytes; 2) Genetic inactivation causing abnormal osteocytes and defective bone mineralization show parallel changes in cementocytes and cellular cementum; 3) Knockout of Sost in mice promotes increased bone from altered osteocyte-osteoblast signaling, and similarly expanded cellular cementum formation suggests common molecular regulation between cementocytes and cementoblasts; 4) In a model of experimentally induced apposition (EIA) in mice used to promote rapid, new cellular cementum deposition, cementocytes exhibited cellular changes and signs of activation, and proteomic analysis of cellular cementum after EIA identified increased intracellular and cell membrane proteins, evidence of cementocyte activation; and 5) Cementocytes express osteoclast signal RANKL in vitro and in vivo its expression is linked to cellular cementum resorption. We hypothesize that cementocytes function in cementum formation and resorption, and this will be tested by two specific aims: 1) Define the requirement for cementocytes in cellular cementum apposition using novel transgenic mice in a model of EIA, combined with proteomic analysis; and 2) Determine the function of cementocytes in cellular cementum resorption using transgenic mice and a mode of orthodontics -induced apical root resorption and by mechanically loading a cementocyte line in vitro for gene analysis. Future Research Plans: Experiments will provide the first definitive data on roles of cementocytes in cementum biology and begin to define molecular signaling pathways involved in cementum apposition and resorption, providing fundamental and completely novel insights into cementum biology that may have clinical relevance in periodontal diseases. Future work will focus on potential molecular interventions to prevent or reverse apical root resorption, and induce cementocytes to promote repair cementum formation.